1. Field of the Invention
The present invention relates to a circuit breaker for use in wiring protection or the like, and particularly relates to a bridging two-contact type circuit breaker in which the opening speed of a movable contact in high-current breaking is increased to enhance the current-limiting performance.
2. Description of the Related Art
FIGS. 9 and 10 show a conventional example of a circuit breaker of this type. FIG. 9 is a longitudinal sectional view, and FIG. 10 is a sectional view taken on line Xxe2x80x94X in FIG. 9. In FIGS. 9 and 10, a pair of front and rear fixed contact 2 and 3 disposed so as to be opposite to each other, and a movable contact 4 for bridging the fixed contacts 2 and 3 are provided for every pole in a molded case 1. The movable contact 4 is pressed onto the fixed contacts 2 and 3 by a contact spring 5 inserted between the movable contact 4 and the molded case 1 to thereby close a conducting path. The fixed contacts 2 and 3 and the movable contact 4 have fixed contact points and movable contact points in their contact portions respectively. A pair of front and rear magnetic drive yokes 6 each of which is made of a U-shaped magnetic substance are disposed such that the opposite end portions of the movable contact 4 are held between the left and right leg portions of the magnetic drive yokes 6 respectively. In FIG. 10, a pair of left and right partition walls la are formed integrally with the molded case 1 to cover the conducting path for every pole. The bottom portion of the molded case 1 is opened, and this opening is closed by a bottom cover 7. The bottom cover 7 supports the magnetic drive yokes 6. In the bottom cover 7, a pair of left and right partition walls 7a are formed for every pole and integrally with the bottom cover 7 to thereby interphase-insulate the magnetic drive yokes 6. Arc-suppressing devices 8 (FIG. 9) are disposed in front and in the rear of the movable contacts 4 respectively.
When an overcurrent flows in the closed state in FIG. 9, a switching mechanism 10 receives a tripping signal from an overcurrent detector 9 and pushes the movable contacts 4 through a push rod 11 so as to detach the movable contacts 4 from the fixed contacts 2 and 3 against the contact springs 5. At that time, an arc is generated between the fixed and movable contact points. However, a magnetic field generated around the conducting path is enhanced by the magnetic drive yokes 6 and interlinked with the arc. Thus, the arc is driven toward the arc-suppressing device 8 by Lorentz force so as to be suppressed. At this time, the vicinities of the fixed and movable contact points are filled with high-pressure conductive gas generated by the arc. Thus, the magnetic drive yokes 6 for every pole are brought into a charged state. When the illustrated circuit breaker is assembled, the fixed contacts 2 and 3, the movable contacts 4, the switching mechanism 10, and so on, are first installed in the molded case 1. Then, the magnetic drive yokes 6 in which an insulating sheet 12 is fitted in advance are inserted from below the molded case 1. Lastly, the bottom cover 7 is fixed to the molded case 1.
In the above-mentioned conventional configuration, the magnetic drive yokes 6 brought into the charged state at the time of breaking are insulated with the partition walls 7a between different phases. However, there is a problem that the vicinities of the contact points are filled with conductive gas with high pressure so that the insulation of the magnetic drive yokes 6 between different poles lacks reliability. Particularly, if the bottom cover 7 is imperfectly fixed to the molded case 1, the partition walls 7a are displaced downward. Thus, there is a danger that the magnetic drive yokes 6 are interphase short-circuited so as to make breaking impossible. In addition, in the above-mentioned conventional configuration, all the magnetic drive yokes 6 in the respective poles are positioned by the bottom cover 7. Therefore, there is a problem that the assembling step is so troublesome that the number of man-hours increases.
It is therefore an object of the present invention to ensure the interphase insulation of the magnetic drive yokes at the time of breaking, and to make assembling easy.
To achieve the above object, according to the present invention, there is provided a circuit breaker in which there is provided for every pole and in a molded case: a pair of front and rear fixed contacts disposed to be opposite to each other;
a movable contact for bridging the fixed contactors; a pair of front and rear magnetic drive yokes made of U-shaped magnetic bodies and disposed to hold opposite end portions of the movable contact between left and right leg portions of the magnetic drive yokes respectively; a gate-like movable contact holder having a pair of left and right leg portions for holding the movable contact movably in an open/close direction, the movable contact holder being guided movably in the open/close direction of the movable contact by the molded case; a U-shaped holder support combined with the movable contact holder slidably in the open/close direction of the movable contact; a pair of front and rear insulating covers formed integrally with the holder support so that each of the insulating covers covers a pair of leg portions of each of the magnetic drive yokes; and a contact spring inserted between the movable contact and the holder support; wherein the movable contact is pressed onto the fixed contacts by the contact spring so as to close a conducting path for each pole, while the movable contact is pressed against the contact spring by a switching mechanism so as to be detached from the fixed contacts when the conducting path is opened; and wherein a lock protrusion formed on one of the movable contact holder and the holder support is engaged with a lock surface formed on the other of the movable contact holder and the holder support so as to bear spring force of the contact spring.
In the invention, since the magnetic drive yokes are covered with the insulating covers, there is no fear that any interphase short-circuit occurs even if the vicinities of the contact points are filled with conductive gas. In addition, the magnetic drive yokes are inserted into the holder supports so as to be supported thereby, while the holder supports are locked in the movable contact holders by spring force of the contact springs so as to be retained by the movable contact holders. Thus, the movable contact mechanism is unitized so that assembling becomes easy. It is preferable that a commutating plate for commutating the movable-contact-side foot of an arc generated between the fixed contact and the movable contact at the time of current breaking is formed integrally with the magnetic drive yokes.